In which order are complement components activated in the classical complement pathway?

In the classical complement pathway, the activation of complement components follows a specific sequence that begins with C1, followed by C2, C4, and ultimately leading to C3.

C1 is the first component to be activated when an antibody bound to an antigen triggers the pathway. C1 consists of a complex that includes C1q, which recognizes the antibody, and C1r and C1s, which are serine proteases that activate the subsequent components. Once C1 is activated, it cleaves C2 and C4.

C2 and C4 are the next components in the sequence, and they work together to form the C3 convertase (C4b2a), which is crucial for the pathway's function. This C3 convertase cleaves C3 into C3a and C3b, providing important opsonic and inflammatory roles. Therefore, the correct order of activation within the classical complement pathway is C1, C2, C4, and C3, which aligns with the answer provided.